1. Field of the Invention
The present invention relates to a thermal image recording apparatus which comprises a thermal print head provided with a heat sink.
2. Description of the prior art
In the thermal printing process, a dye-bearing donor ribbon is brought into contact with a dye-receiving print sheet located at a print zone. The donor ribbon can be advanced in one direction and thermal printing is effected by contacting the donor ribbon with a multi-element print head which spans the ribbon in a direction transverse to the direction of ribbon travel. The print head typically comprises a linear array of closely spaced resistive heating elements, each being independently addressable by an applied voltage to heat that portion of the donor ribbon directly opposite and thereby cause dye to transfer from the donor ribbon to the print sheet. To maintain intimate contact between donor ribbon and print sheet during this printing operation, the donor ribbon and print sheet are partially wrapped over the surface of a rotatably driven platen roller.
The degree or density of picture element formation on the print sheet depends on the temperature of the heating elements and on the temperature of the print head itself. The electric energy applied to the heating elements is kept within a prescribed range by a control circuit. The temperature of the print head itself, on the other hand, is kept under control by dissipation of the heat accumulated during printing by means of a heat sink. To that end, a stream of cooling air produced by a blower is directed over the fins provided for that purpose on the heat sink.
The blower for cooling the print head is located within the housing of the image recording apparatus. This has the disadvantage that the air heated by the heat sink circulates within the image-recording apparatus, thereby increasing the temperature inside this apparatus and reducing the life of various components, especially electric components.
It has been proposed to overcome this problem by providing a thermal-image recording apparatus with duct means for guiding the stream of air used for cooling heat sink to the outside of the image-recording apparatus. This technique is disclosed in U.S. Pat. No. 5,053,792.
The mentioned solution does not solve, however, the problem of air circulation through the interior of the image-recording apparatus since-all the air drawn by the ventilator means passes through a substantial part of the interior of the apparatus whereby environmental dust and other particles carried by the air become deposited eventually on the surface of the print head, on rotating parts, on lubricated surfaces, etc. This deposition requires a regular maintenance of the apparatus as well as risks of causing damage to delicate components.